Tire treads for pneumatic tires typically have running surfaces of a singular rubber composition and consistent physical properties across the face of the tread intended to be ground contacting.
Often the tire tread may be of a cap/base construction composed of an outer tread cap layer presenting the running surface of the tire and an underlying tread base layer as a transition between the tread cap layer and the tire carcass. The tread cap layer itself may be of a lug and groove configuration with the outer surface of the lugs, including lugs in a form of ribs, themselves presenting the running surface of the tire tread. Such overall tire tread cap/base construction is well known to those having skill in such art.
For example, an all-season tire tread cap layer may be of an individual rubber composition designed to present a tread running surface for a balance of a combination of wet traction, cold weather winter traction (for snow and/or ice), dry handling, and resistance to tread wear properties.
However, optimizing one or more individual tread properties such as, for example, wet traction, cold weather winter traction, dry handling and resistance to tread wear properties typically requires a compromise of one or more physical properties.
Tires have been heretofore proposed which have circumferentially zoned treads for various purposes including a desire to present a plurality of individual running surfaces with various characteristics from one tire tread. For example, see U.S. Pat. Nos. 4,319,620, 4,385,653, 6,415,833, 6,474,382, 6,540,858 and 6,959,744; U.S. Patent Application Nos. 2002/0033212, 2004/0112490 and 2005/0167019; European Patent Publication Nos. 0341187, 0662396, 0839675 and 1308319; WO99/01299; and Japanese Patent Publication Nos. 2001/047815 and 85/60135309.
For this invention, it is desired to present an outer tread cap layer with a running surface composed of two or three individual circumferential load-bearing portions, namely said primary tread cap portion and said one or two individual lateral tread cap portions.
In one instance, said tread cap layer is composed of two individual circumferential tread zones, or portions, namely said primary tread cap zone and an adjoining outboard lateral tread cap zone.
In another instance, said tread cap layer is composed of three circumferential tread zones, or portions, namely said primary tread cap zone and adjoining individual outboard lateral tread cap zones.
For this invention, a primary circumferential tread cap zone is provided which constitutes at least sixty percent (e.g. from about 60 to about 90 percent) of the axial width of the tread running surface intended to be ground contacting. The remainder of such tread running surface is comprised of said one or two individual lateral circumferential tread cap zone(s) which are individually positioned next to and axially outward from said primary tread cap zone, wherein said lateral circumferential tread cap zones may be of the same or different widths.
Therefore, in one instance, such tread cap may be of an asymmetrical configuration in the sense of the aforesaid primary tread cap zone and one outboard lateral tread cap zone or the aforesaid primary tread cap zone and two individual lateral tread cap zones of unequal widths for the tread running surface. In practice, for the tread cap layer with a single outboard lateral circumferential tread zone, it is intended that the tire is mounted on a rim to form a tire/rim, or wheel, assembly, with said outboard lateral tread cap zone positioned axially outward (away from the vehicle) insofar as the associated vehicle is concerned. Where the tread cap contains two individual lateral tread cap zones, one lateral tread cap zone is to be said outboard lateral tread cap zone and the other is to be an inboard lateral tread cap zone (inboard with respect to the vehicle with which the tire is to be associated). Accordingly, at least one of said lateral tread cap zones is designed to be an outboard lateral tread cap zone in a sense of intended tire orientation on an associated vehicle and in such manner provide an additional (outboard positioned) control element, or zone, to the primary tread cap zone of the tire with the term “outboard” referring to the axial outer position of the lateral tread cap zone of the tire tread.
Accordingly, said asymmetrically configured zoned tread running surface is not composed of a central or other circumferential tread zone centered over the centerline, or equatorial plane, of the tire.
Historically, various discontinuous (non-woven) fibers have also been used, particularly to enhance both stiffness and modulus of rubber vulcanizates. For example, discontinuous cellulose fibers, including such fibers with high aspect ratios, have been used as dispersions thereof in rubber as disclosed for example in U.S. Pat. Nos. 3,697,364, 3,802,478, and 4,236,563.
Various carbon fibers, including short fibers and carbon cords, have been suggested for use in various rubber compositions, including tire treads. For example, see U.S. Pat. Nos. 5,323,829 and 5,718,781. A multi-filament carbon yarn has been suggested for reinforcement of rubber compositions in which the yarn is coated with a composite of resorcinol-formaldehyde reaction product, a vinylpyridine-styrene butadiene terpolymer, an acrylonitrile-butadiene copolymer and urea. For example, see U.S. Pat. Nos. 6,077,606 and 6,350,492.
For this invention, short, discontinuous carbon fibers are used for the rubber reinforcement which have been coated (pre-coated) with an adhesive composition (to aid, or enhance, adhesion of the carbon fibers to the elastomer) comprised of a vinylpyridine-styrene butadiene terpolymer, an acrylonitrile-butadiene copolymer and optionally urea (RFL adhesive coating).
In practice, such carbon fibers are preferably in a form of a cord comprised of twisted (cabled) carbon filaments having a carbon filament count (number of carbon filaments in the cord) preferably within a range of from about 1,000 to about 48,000. The average filament length (cord length) is preferably within a range of from about 1 mm (millimeter) to about 10 mm, optionally in a range of from about 3 mm to about 5 mm and an average diameter within a range of from about 2 to about 15 microns.
Representative of an adhesive coated carbon yarn is a carbon multi-filament yarn impregnated with a resorcinol-formaldehyde-rubber composite. The adhesive coated multi-filament yarn may then chopped into the short yarn fibers for use in this invention.
Such resorcinol-formaldehyde composite may be, for example, comprised of, and the product of, a resorcinol, formaldehyde, vinylpyridine/styrene/butadiene terpolymer latex and acrylonitrile/butadiene copolymer latex blend, which may optionally include urea. After application to and encapsulation of the carbon multi-filament yarn, the blend is dried and cured at an elevated temperature in a sense of allowing the latices to dry and the resorcinol and formaldehyde to react and thereby form a resin within the resultant rubber and to thereby form the resorcinol-formaldehyde resin-rubber composite coated carbon filaments. The resorcinol and formaldehyde react with each other in situ within the latex binder to form an adhesive resin. The rubber copolymer(s), in one aspect, form a resultant binder for the resin. For an example of such coated carbon fibers, see U.S. Pat. No. 6,077,606. For convenience, said adhesive coating may be referred to herein as an RFL (resorcinol-formaldehyde-latex) adhesive coating. If desired, the carbon multi-filament yarn may be pre-treated by impregnating the yarn with an epoxy resin prior to its impregnation with the RFL composite in order to provide enhanced bonding of the filaments to the said RFL composite.
The yarn comprised of the RFL adhesive coated (encapsulated) carbon filaments is then chopped into short filamentary lengths and the coated filaments separated into short fibers, all by suitable means, for blending with and dispersing into the rubber composition.
A representative example of a multi-filament carbon yarn for such RFL treatment, or coating, is, for example, T700GC™ from Toray Industries. In practice, said carbon yarn may contain, for example, a range of from about 1,000 to about 48,000 carbon filaments.
Historically, tire treads have heretofore been suggested having running surfaces composed of three longitudinal portions namely, two black colored lateral portions and a non-black colored central portion located between the two black portions, wherein the lateral black colored portions have wear resistant properties virtually identical to the central colored portion (EP 0 993 381 A3, FR 2765525 and WO 99/01299 patent publications).
In U.S. Pat. No. 5,225,011 a tire is presented having a tread composed of a center rubber composition and side rubbers (its FIG. 1) positioned directly onto a tire carcass belt without a tread base transition layer. The center rubber is required to be limited to either natural rubber or a natural rubber/styrene-butadiene rubber blend. The center rubber contains a carbon black of large iodine absorption number of at least 100 mg/g, silica and silane coupling agent and the side rubbers are required to be of a different rubber composition.
In European patent publication number EP 864,446 A1 a tire is presented having a tread (its FIG. 2) with a central portion (B) and side portions (A) positioned directly onto a tire carcass belt without a tread base transition layer. The side portions are carbon black rich and the central portion is silica rich, wherein the silica content of the central portion (B) is at least 20 percent higher than in the side portions (A).
For the zoned tread cap layer of this invention, by requiring the tread cap zones to be capable of being load-bearing, it is meant that each of the distinct tread running surface tread cap zones extend radially inward from the outer surface of the tread cap layer to the underlying tread base layer rubber composition so that the load on the tire may be communicated by the tread cap layer zones to the transitional tread base layer instead of directly to the remainder of the tire carcass itself.
The term “running surface”, or “total running surface”, of the tread cap layer, unless otherwise indicated, means the total outer surface of such tread cap layer which is intended to be ground-contacting, including such outer surface of the tread cap layer which is intended to be intermittently ground-contacting and the included space, or span, across the opening of any tread grooves contained in such tread cap layer at the running surface level. When a tread cap zone is referenced herein as spanning a percentage of total running surface of the tread cap, unless otherwise indicated, such span extends axially, or laterally, across such running surface (e.g. basically, in a direction substantially perpendicular to the equatorial plane (EP) of the tire).
In the description of this invention, the terms “rubber” and “elastomer” may be used interchangeably, unless otherwise provided. The terms “rubber composition”, “compounded rubber” and “rubber compound” may be used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art. The terms “cure” and “vulcanize” may be used interchangeably unless otherwise provided: In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer.